1. Field of the Invention
The present invention relates to a free-space optics communication apparatus and an optical detection apparatus, which allow apparatuses, which are arranged opposite to each other and spaced with a predetermined interval, to perform bidirectional information communication.
2. Description of Related Art
In Japanese Patent Application Laid-Open No. H05-133716, the free-space optics communication apparatus comprising an optical axis correction unit which detects an incident direction of light flux emitted from a counterpart apparatus and emits its light flux toward the incident direction is disclosed. A characteristic structure of the conventional free-space optics communication apparatus is shown in FIG. 12. The free-space optics communication apparatus has a light-transmitting optical system and a light condensing optical system as shown in FIG. 12, and these two apparatuses having substantially the same structure are arranged opposite to each other and spaced from each other to perform bidirectional communication.
Laser lights emitted from a laser diode 1, which are linearly polarized in the direction perpendicular to the paper surface of the same figure, are converted to substantially parallel light flux by a lens unit 2 having a positive power and are reflected at a boundary surface (a polarized light separating surface) of a polarization beam splitter 3. And the reflected light is reflected again by a mirror 4 of an optical axis direction varying section 10, and then transmitted as sent light LA from an apparatus A to an apparatus B which is not shown.
The received light LB which is emitted from the counterpart apparatus and is incident on the main apparatus is reflected by the mirror 4 and transmitted through the polarization beam splitter 3 to reach a received light splitting mirror 5. In this case, about 90% of the received light LB is transmitted through the received light splitting mirror 5 and is condensed at a main signal detection light-receiving element 6 by a lens unit 7 having a positive power. And the rest of about 10% is reflected by the received light splitting mirror 5 and is condensed at a position detection light-receiving element 8 by a lens unit 109 having a positive power.
An optical element on whose attached surface a multi-layered thin film is deposited is used as the polarized beam splitter 3. This multi-layered thin film is configured so that S polarized light is reflected and P polarized light is transmitted. In order to attain the most efficient light transmission and reception using the polarized beam splitter 3, it is preferable to have the received light LB being P polarized light when the sent light LA is S polarized light.
Moreover, in order to perform the most efficient light transmission and reception by arranging the light-transmitting apparatus and the light-receiving apparatus having the same structure arranged opposite to each other, it is preferable to have an “optical axis on the beam splitter side” 13 disposed to be inclined behind the paper surface of the same figure which is a common optical axis for transmission and reception, such that the polarization direction of the sent light LA and that of the received light LB are perpendicular to each other when these two apparatuses are arranged to face each other.
In addition, in order to perform communication having a large amount of transmitting information, small elements such as an avalanche photodiode, which has a diameter of an effective light-receiving area less than 1 mm, should be employed for a main signal detection light-receiving element 6. Accordingly, in order to dispose the received light LB within the effective light-receiving area of the main signal detection light-receiving element 6, an angle of the mirror 4 is adjusted so that the position of the main signal detection light-receiving element 6 is aligned with that of a position detection light-receiving element 8. Thus, the optical axis of the received light LB is arranged to be substantially at the center of the position detection light-receiving element 8.
In this case, in order to effectively transmit the sent light LA toward the counterpart apparatus, the optical axis of the sent light LA, namely, the laser diode 1 preferably coincides with the center of the position detection light-receiving element 8.
Position deviation information of a spot SP, formed on the light-receiving surface of the position detection light-receiving element 8 by the received light LB, is sent by a signal processing section 11 to an optical axis direction control section 12 as an optical axis deviation correction signal, and the optical axis direction control section 12 sends an optical axis direction changing signal to an optical axis direction varying section 10.
Further, based on this optical axis direction polarized light signal, the angle of the mirror 4 is adjusted so that the optical axis of the sent light LA coincides with that of the received light LB.
Such control is continued during communication, and the bidirectional communication apparatuses, which are disposed opposite to each other and spaced with a predetermined interval, are corrected mutually such that the optical axis of the received light LB transmitted from the counterpart apparatus coincides with the center of the position detection light-receiving element 8. Thus, the optical axis of the transmitted light LB can be arranged to coincide with that of the received light LA.
FIG. 13 shows a structure of a position detecting element in accordance with the related art. A four-division sensor 13 which is divided into four elements 14 is generally employed as the position detection light-receiving element 8. However, when such a light-receiving element is used for the position detection light-receiving element 8, it is preferable to allocate a proper area to the spot SP of the received light LB so as to repress a rapid change of a sensor output when the light crosses a separation zone 15 between the separated elements. Accordingly, the position of the light-receiving surface is generally set at a position defocused from the condensing point.
However, in the free-space optics communication apparatus for performing light transmission and reception in the atmosphere, the transmission is affected by fluctuation phenomena of transmitted beams caused by vibrations of an installed position of the apparatus or fluctuations of the atmosphere. These atmospheric fluctuations may be classified into a macro-fluctuation in which the whole transmitted light fluctuates and a micro-fluctuation in which the intensity distribution of the transmitted light fluctuates. In this case, although the macro-fluctuation of the atmosphere may be overcome along with the vibration associated with the installed position, another method should be taken into consideration to deal with the micro-fluctuation.
FIG. 14 is a conceptual diagram in which the micro-fluctuation of the atmosphere is modeled. Reference character W denotes a spread when the received light LB emitted from the counterpart apparatus reaches the main apparatus. The atmosphere is a non-uniform medium which has a convection current caused by pressure or temperature differences and whose refractive index varies not only in space but also in time. As a result, the received light LB is diffused, and a portion W1 having a strong intensity and a portion W2 having a weak intensity appear in the spread W.
This intensity distribution varies as a function of time, so that W2 is seen as fluctuating in the spread W where the sent light LA is diffused. This is referred to as the micro-fluctuation of the atmosphere, and this fluctuation occurs randomly. In the free-space optics communication apparatus of the related art, the light-receiving surface of the position detection light-receiving element 8 is disposed at a position defocused from the condensing point, so that, in a state of the micro-fluctuation of the atmosphere as described above, the spot SP having a proper area on the light-receiving surface does not have a uniform intensity distribution and the distribution of the light intensity at a beam taking inlet M into an apparatus corresponding to an entrance pupil is transmitted as it is. (See FIG. 15.)
FIG. 16 shows a feature of the spot SP formed with light flux collected from the beam taking inlet M. The spot SP of a diameter T has a portion P1 with a strong intensity (unshaded) and a portion P2 with a weak intensity shown with oblique lines. The center of gravity of spot light PC, which is different from the center of the light flux BC, is judged to be an optical axis, and there occurs a deviation toward the optical axis direction of the sent light LA by an angle associated with the amount of position deviation S thereof. As a result, the sent light LA is deviated from the counterpart apparatus, which causes the problem in that communication cannot be performed.
In addition, the four-division sensor has been described up to now, but the defocusing problem may be avoided by repressing a rapid output change by employing a sensor referred to as a position sensitive detector (PSD) such as a semiconductor image position detecting element, which does not have the above-mentioned separation zone. However, in an apparatus whose communication distance ranges from several tens meters to several kilometers, it is difficult to dispose the position detection light-receiving element 8 at a place nearest to the best position.
Moreover, such apparatuses should be adjusted to have the following position relationship between the laser diode 1 and the lens unit 2. In case of a short distance, the beam is made broad so that the sent light LA does not adversely affect on human eyes. In case of a long distance, the beam is made narrow so that the beam energy securely reaches the counterpart apparatus. Therefore, the defocusing method cannot be avoided on the position detection light-receiving element 8.
Accordingly, the defocusing method should be taken into consideration in the case of PSD. The PSD should detect the center of gravity position of the spot light even in the defocused state, and there is no difference from the case of the four-division sensor.
The present invention is made to overcome the above-mentioned problems, and it is an object of the present invention to provide a free-space optics communication apparatus and an optical detection apparatus, which are capable of performing stable communication by reducing an optical axis deviation correction error regardless the non-uniform intensity distribution of the received light due to the micro-fluctuation of the atmosphere.